Fresnel Card Lens

From WildernessWiki.org

A fresnel card lens is a magnifying glass the size and shape of a credit card. Such a lens is excellent for wilderness First Aid Kits and Survival Kits since it is extremely thin and lightweight, yet will allow splinters, small wounds, and insects to be easily viewed. A fresnel lens is also a good tool for viewing maps with small print. In a survival situation, a fresnel lens can be used to start a fire by focusing sunlight onto a tinder.

Fresnel lenses are frequently available for free in optometrists' offices.